Q. Years ago, I was taught to begin an oil painting with a charcoal sketch and then to fix it by spraying on an acrylic coating. Can I do this on a canvas thats already been primed with an acrylic coating from the factory, or will this adversely affect the bonding surface for my oils?George Thompson Birmingham, AL

A. Assuming youre applying your fixative correctly, this process shouldnt threaten the bonding of your oil paint to the acrylic ground, whether or not the canvas is factory-primed. If you overapply it, however, you risk sealing off your ground layer, which could cause adhesion problems. Oil paint needs to absorb slightly into its ground to adhere securely, and the more fixative you apply, the more it becomes like a varnish, preventing any penetration. In the extreme, the effect would be something like the adhesion problems often seen with oil painting on glass.

Remember that while fixatives help reduce the chances of smudging theyre not designed to completely withstand all mechanical action, such as the more vigorous passes of a paintbrush. If your technique is rough, then the act of painting may disturb your charcoal underdrawing with or without fixative. If you havent been bothered yet by the accidental incorporation of charcoal particles into your oil paints, and if youre worried about adhesion, then perhaps you wouldnt miss the fixative if you omitted it altogether. Try painting directly over your drawing or choose a drawing medium, like pencil, thats less likely to smudge.

On the other hand, if you plan to allow elements of your sketch to remain visible in your final composition then fixative remains a good precaution against future abrasion. Apply the fixative lightly to keep from soaking the charcoal, which, like pastel, is a medium of dry particles that loosely bind to the surface. A light spraying of fixative, which is a low concentration of a resin (such as acrylics) in a solvent, is enough to hold the charcoal in place, while a heavy coat would defeat the charcoals delicate purpose. Again, this shouldnt affect the adhesion of your oil paint, regardless of whether the canvas is primed.

Finally, beware of the potential for discoloring the ground with the fixative if part of your sketch is to remain visible. While acrylic fixatives shouldnt cause any discoloration, those made with shellac or other natural resins most certainly will.